So far, there have been no synthetic pulps which can be obtained more economically than natural pulp. Up to now, all techniques for obtaining synthetic pulps rely on a conventional fiber-making process which is comprised of various steps such as dissolving polymers, spinning, coagulating, drying, drawing as-spun filaments, recycling solvents, and additional measures which serve to reduce the resulting pollutants. Thus, the economical load undertaken to produce synthetic pulps is inevitably very large.
Acrylic fibers have been used in materials for clothing, as well as more recently, in industrial materials, for example, as a substitute fiber for asbestos, a heat insulating and resisting fiber, a cement reinforcing fiber and the like. Acrylic fibers used in industrial materials should be produced in the form of a short fiber.
It is well known that PAN exists in the form of a conglomerated particle in which the molecular chains of PAN are twisted into an irregular helix due to the flexibility of skeleton chains and the strong polarity of nitrile groups in the side chains thereof (See F. G. Frushour et al., Handbook of Fiber Science and Technology, Volume IV, Fiber Chemistry, pp. 171-370, ed. by M. Lewin and E. M. Pearce, Marcel Dekker, Inc., 1985). If a strong polar solvent, such as dimethylformamide, dimethylacetamide, dimethyl-sulfoxide, or an aqueous NaSCN solution, an aqueous ZnCl.sub.2 solution, or an aqueous HNO.sub.3 solution, is added to such polymers, the nitrile groups attract the solvent molecules to couple therewith, and thereby are separated from each other to form a fluid solution. The resulting solution is extruded through the microholes of a spinning die. After the solvent is removed, PAN is solidified to form fibers. However, the molecular chains in the solidified PAN still exist in the conglomerated state, while maintaining the form of a non-oriented lump.
Therefore, although the filaments take the form of a fiber immediately after spinning, when the solvent is removed and the filaments are dried, the PAN molecular chains in the filaments reconglomerate to form a non-oriented lump since the internal molecular chains of the resulting filaments are not oriented at all. Accordingly, it is necessary to draw the resulting filaments in a high ratio (5 to 30, or more) in order to obtain a complete fibrous structure in which the molecular chains are arranged in parallel with the fiber axis. As the filaments are drawn, the non-oriented, conglomerated PAN molecular chains disentangle, extend, and are arranged in parallel with each other, thereby forming fibers having an extended chain crystal region. Therefore, the drawing step is indispensable in the prior techniques for producing fibers since the fiber structure, in which most of the molecular chains are oriented in parallel with the fiber axis, can be obtained only by the drawing operation.
Conventionally, short fibers have been produced in the form of a staple by first forming a long fiber through a solution spinning process using a solvent, and then drawing and cutting the resulting long fiber into staples.
However, the solvents used are now recognized as contributing to environmental pollution. Moreover, the complicated steps of extracting, recovering and purifying the solvents, as well as the maintenance of anti-pollution facilities, increase production costs. Further, the filament thus formed appears to be a fiber, but it still remains substantially unoriented. Accordingly, the filaments thus obtained must be subjected to drawing in a high stretch ratio of between 5 to 30 in order to afford a complete fibrous structure in which the molecular chains are arranged in parallel with the axis of the fiber. This drawing may also increase production costs.
In the case of an acrylic fiber having large surface areas, the process for manufacturing the same involves the more complicated steps of providing a spinning solution, spinning the solution, solidifying the spun filament, removing and recovering the solvent used, drawing and cutting the filament, fibrillizing the resulting fiber, and so forth.
In general, the acrylic fibers prepared by the prior art techniques are inadequate as spun yarns due to their poor elasticity and slippery surface. Further, they are not satisfactory in terms of their reinforcing, heat insulating, and binding properties which are required of industrial materials.
In order to solve the problems mentioned above, it has been suggested to prepare a PAN hydrate using water in place of the hazardous solvents. For example, U.S. Pat. No. 2,585,444 teaches that PAN fibers can be formed by heating a hydrate of PAN, containing 30% to 85% water (by weight), to its melting temperature or higher to give a melted fluid, and then spinning the resulting melt. U.S. Pat. Nos. 3,896,204 and 3,984,601 disclose a process for the production of fibers which comprises heating a mixture of PAN and about 20% to 30% water (by weight) to a temperature ranging from 175.degree. C. to 205.degree. C., to give an amorphous melt, and spinning the resulting melt to form filaments which are then drawn to five times their original length. It is also described in the above patents that when the content of acrylonitrile in PAN is as low as 80%, spinning can be carried out at a temperature range between 140.degree. C. and 170.degree. C.
U.S. Pat. Nos. 3,991,153 and 4,163,770 disclose a process for the production of fibers which comprises spinning a PAN hydrate containing between 10% to 40% water (by weight) at the melting temperature or higher, that is, the temperature range at which the melt of an amorphous single-phase is formed, and then drawing the filaments in a ratio between 25 to 150 in a pressure vessel.
As mentioned above, the prior art processes typically involve the steps of forming and spinning a PAN/H.sub.2 O melt. However, since the spinning is carried out within the temperature range at which the melt exists in a random amorphous state, fibers, in which the molecular chains of PAN are highly oriented, cannot be obtained without a subsequent step of drawing in a high draw ratio.
Further, U.S. Pat. Nos. 3,402,231, 3,774,387, and 3,873,508 disclose a fiber production process which comprises forming a PAN/H.sub.2 O melt by heating a mixture of PAN with 50% or more water to about 200.degree. C., and then spinning the resulting melt to produce fibers. However, such large amounts of water contained therein, and such high temperatures, provide a random, amorphous PAN/H.sub.2 O melt. In addition, the PAN filaments extruded from the melt have a fiber profile, but are in reality, no more than non-oriented, continuous extrudates which do not possess any oriented molecular chains nor fibrous structures.
As mentioned above, the prior art techniques for spinning a PAN/H.sub.2 O melt are based on a commonly used process which comprises forming an amorphous melt of the PAN/H.sub.2 O mixture by using a large amount of water and temperatures higher than the melting temperature of the PAN/H.sub.2 O mixture, or by increasing the content of the comonomers, spinning the resulting amorphous melt to form filaments, and then, drawing the resulting filaments in a high draw ratio to form fibers.
In the conventional techniques for producing filaments, an amorphous melt is obtained by heating the PAN/H.sub.2 O mixture to the temperature at which the crystalline phase is broken down, in order to lower the viscosity of the resulting melt for easy spinning operation. Thus, a parallel arrangement of the PAN molecular chains cannot be achieved without a separate step of drawing in a high draw ratio. Furthermore, fibers, for use in making pulp, and which have highly orientated molecular chains, could only be prepared by a process which comprises the following complicated steps: preparing a stock solution in which PAN is dissolved in a solvent, spinning, solidifying, removing and recovering the solvent, drying, drawing, cutting, and fibrillating.
Given the above, we, the present inventors, have further investigated a two-component system comprising PAN and water (hereinafter referred to as the "PAN/H.sub.2 O mixture") and unexpectedly found that the PAN/H.sub.2 O mixture absorbs the heat of fusion to form a melt of an amorphous, single phase at the melting temperature of the mixture.
The melted single phase, even if cooled to below the melting temperature, is not solidified and maintains its supercooled state until the temperature is cooled to a selected temperature range. When further cooled to below the solidifying temperature (Tc), the melted single phase is solidified and is returned to its original state. However, when the PAN/H.sub.2 O melt is cooled to form the supercooled state at a temperature below the melting point, the single phase PAN/H.sub.2 O melt forms a metacrystalline phase having a molecular order. Unlike the amorphous melt formed at temperatures above the melting point, the physical properties of the metacrystalline phase are similar to those of a liquid crystal.
That PAN, together with water, forms a metacrystalline phase, in which a liquid crystal structure can be formed by applying only a small shear force at a temperature below the melting temperature of the PAN/H.sub.2 O mixture, was first found by the present inventors. This surprising phenomenon makes it easy to produce PAN with a molecular orientation upon simple extrusion, which will be discussed in detail hereinafter. It appears that in the melted metacrystalline phase, the PAN molecular chains, together with the water molecules, form innumerable fine units of a regular phase having the molecular structure of an ordered chain form. The PAN molecular chains in the melted metacrystalline phase have a self-orientating property. Thus, if directional shear forces are applied to the PAN molecular chains in the melted metacrystalline phase, the PAN molecules easily form a highly-oriented fibrous structure. In other words, if the PAN/H.sub.2 O melt in the melted metacrystalline phase is extruded, the extended PAN molecular chains align in parallel with each other and the water is spontaneously expelled from the system. As the water is expelled, the PAN molecules are extended so that a fiber structure is formed, thereby producing highly-oriented fibers even without a separate drawing process.